In pressure measuring technology, absolute-pressure, relative-pressure and differential-pressure measuring cells are used. In absolute-pressure measuring cells, a pressure to be measured is detected in absolute form, that is, as a pressure difference compared to a vacuum. With a relative-pressure measuring cell, a pressure to be measured is picked up in the form of a pressure difference compared to a reference pressure, such as a pressure that prevails where the sensor is located. In most applications, at the location of use this is the atmospheric pressure. Accordingly in absolute-pressure measuring cells, a pressure to be measured is detected with reference to a fixed reference pressure, that is, the vacuum pressure, and in relative-pressure measuring cells, a pressure to be measured is detected with reference to a variable reference pressure, such as the ambient pressure. A differential-pressure measuring cell detects a difference between a first and a second pressure applied to the measuring cell.
There are pressure measuring cells on the market, having: a base body; a membrane, connected to the base body, forming a measuring chamber, which in operation executes a deflection that is dependent on a pressure to be measured; and an electrode, disposed in the measuring chamber on a side of the base body toward the membrane, which electrode together with a counter-electrode applied to the membrane forms a capacitor, whose capacitance is a measure for the deflection of the membrane.
In such pressure measuring cells, an electrical connection of the electrode can be accomplished either through an interstice between the membrane and the base body, or through the base body. In the first instance, a joining material, by which the membrane and the base body are connected to one another, must be an electrical insulator. Contacting through the base body is to be preferred, since it leaves the connection between the base body and the membrane unimpaired and thus leaves it tight and mechanically stable, and since it involves no limitation in the selection of the joining material.
In conventional contacting through the base body, a metal contact pin is inserted into a bore that penetrates the base body and is compressed on the end, for instance with an arbor. As a result, the contact pin is mechanically fixed, and an electrical contact point with the electrode of the base body is created.
In most applications, this method furnishes very good results and can be performed quickly and economically. However, applications exist in which this method of compressing is disadvantageous.
The compressing does not achieve much tightness. Vacuum tightness of the kind required in absolute-pressure measuring cells is unattainable. Consequently, especially with absolute-pressure measuring cells, a separate sealing of the through-connection is necessary.
Because of the compressing, an inside face of the measuring chamber in the region of the bore and the metal pin is uneven and has recesses, such as gaps or indentations.
These irregularities in the inside face cause problems whenever the inside face is to be used as a membrane bed, to which the membrane conforms in the event of an overload. Irregularities in the membrane bed in the event of an overload can lead to permanent changes in the membrane that can later cause grave errors of measurement or even complete failure of the pressure measuring cell.
Geometric irregularities, in particular gaps, in the region of the electrical contact point can lead under some circumstances to elevated transition resistances, with attendant disadvantages in picking up the measured value.
Further problem arise if the measuring chamber is to be filled with a pressure mediator fluid. In fluid-filled pressure measuring cells, preferably only a very slight volume of fluid is used, since thermal expansion of the fluid from temperature must be kept as slight as possible. It is correspondingly important for the measurement precision that the volume in which the liquid is placed be as constant as possible. Dents, gaps or other forms of recesses that a fluid volume of unknown size can penetrate over the course of time must absolutely be avoided. In differential-pressure measuring cells, it is especially important not only that the volume be as constant as possible but also that as much as possible the same quantity of fluid is present in both halves of the differential-pressure measuring cell. Unequal fluid quantities result in unequal temperature courses, which have a direct effect on the measurement precision.